1. Field of the Invention
The present invention relates to a fixing unit that fixes a transferred toner image on a recording medium by applying heat and pressure, an electro-photographic image forming apparatus having the same, and a printing method thereof.
2. Description of the Related Art
Generally, electro-photographic image forming apparatuses irradiate light on an equipotentially charged photoconductor to form an electrostatic latent image thereon. The electrostatic latent image is developed with toner having a particular color to form a toner image. The toner image is transferred onto a recording media directly or via an intermediate transfer belt. The toner image transferred onto the recording medium adheres to the recording medium by an electrostatic force. The toner image is permanently fixed on the recording medium after a fixing unit applies heat and pressure thereto.
The fixing unit includes a heat roller and a pressure roller. The two rollers are engaged with each other and form a fixing nip therebetween. When the heat and pressure are applied to the recording medium while the recording medium passes through the fixing nip, the toner starts to melt, thereby becoming fixed on the recording medium. The toner image on the recording medium comes in contact with the heat roller. The toner melted in the process of fixing tends to adhere on the surface of the heat roller, in addition to the recording medium. When a stiffness of the recording medium is large enough to get over an adhesive force between the toner and the heat roller, the recording medium is not wound around the heat roller. Otherwise, the recording medium is wound around the heat roller, and thus a paper jam occurs. To prevent this, a separation claw is provided. The separation claw comes in contact with the surface of the heat roller. A front end of the recording medium is separated from the heat roller when it is snagged by the separation claw. The surface of the heat roller may be damaged by the separation claw, because the separation claw continuously contacts the heat roller.
The stiffness of the recording medium varies depending on the property of the recording medium. The property may be a thickness, or a texture orientation of a paper recording medium. When a thick recording medium is used, the recording medium is easily separated from the heat roller due to its large stiffness, but the fixing property is not good because of poor heat transfer. When a thin recording medium is used, or the texture orientation of the recording medium is arranged crosswise, the stiffness is small, and thus the recording medium is likely to be wound around the heat roller. Additionally, an image is differently fixed to or separated from the recording medium depending on an image density.
Accordingly, a need exists for an image forming apparatus having an improved fixing unit that improves fixing an image onto a recording medium and substantially prevents the recording medium from adhering to the fixing unit.